1. Field of the Invention
The present invention relates to a diagnostic ultrasound Doppler technology enabling effective diagnosis of ischemic cardiac diseases including myocardial infarction and angina, and left ventricular dilation including hypertrophic cardiomyopathy. More particularly, this invention is concerned with tissue Doppler imaging (TDI) in which a Doppler imaging method is used to detect motion information of a cardiac muscle (cardiac wall) or vascular wall, and the results of computation are displayed in color.
2. Description of the Related Art
Currently, quantitative assessment of cardiac or vascular functions is essential to diagnosis of cardiac diseases. Various methods of diagnosis have been attempted.
Ultrasonic diagnosis that is one of such methods is often achieved by observing a real-time B-mode tomographic image of, for example, the left ventricle (the left ventricle is regarded as a center of assessment of cardiac functions). Through observation, a grave symptom of an ischemic cardiac disease, left ventricular dilation, or the like can be diagnosed to some extent.
In recent years, more advanced and specialized diagnostic methods have been proposed. For example, there is left ventricular wall motion analysis dedicated to diagnosis of ischemic cardiac diseases. This analytic method measures change in thickness of a cardiac muscle between a left ventricular systole and diastole, and a region exhibiting a small change in thickness is diagnosed as a "region with a deteriorated contracting ability," that is, an "ischemic region." Various algorithms have been devised for this analysis. Whichever of the algorithms is adopted, a B-mode tomographic image is used to trace the contour of the left ventricular endocardium or epicardium at an end systole and end diastole. The trace information is used for measurement.
Stress echo imaging is also known as a method for diagnosing myocardial infarction. This diagnostic method stresses the heart through exercise, by administering a drug, or by applying an electrical impulse. Ultrasonic tomographic (B-mode) images of the heart produced before and after the stressing are recorded. Thereafter, the images produced before and after the stressing are displayed side by side on one monitor and are compared with each other in terms of change in thickness of a cardiac muscle between a cardiac systole and diastole (cardiac muscles normally thicken during a systole). Thus, a region with infarction can be specified.
However, the foregoing diagnostic methods have various drawbacks such as those described below.
To begin with, when a B-mode tomographic image is viewed and observed for diagnosis, because a monitor screen displays a mere real-time image, it is hard to determine hypofunction of an organ or discriminate an abnormal tissue. Even for a physician having abundant experience in ultrasonic diagnosis, it is practically hard to acquire detailed information of a region with a locally-deteriorated contracting ability deriving from an ischemic cardiac disease, or of left ventricular dilation. Moreover, the left ventricular wall motion analysis is a diagnostic method dedicated to ischemic cardiac diseases, and therefore lacks in generality.
Under these circumstances, a first relatively useful approach is an approach using a tissue Doppler imaging (TDI) system. Using this system, a Doppler signal is extracted from an echo signal on which the motion of a tissue such as a cardiac muscle is reflected, and motion velocity information resulting from frequency analysis of the Doppler signal is displayed as a color tomographic image. The color tomographic image is displayed while being updated sequentially frame by frame, or displayed as a frozen image.
Furthermore, as a second approach, an approach using a diagnostic ultrasound system in which tracing the contour of a tissue and coloring are combined for imaging has been known in the past (for example, Roberto M. Lan et al., "Echocardiographic Qantification of Regional Left Ventricular Wall Motion With Color Kinesis", 1877-1885, Vol. 93, No. 10, May 15, 1996). Taking a cardiac muscle for instance, the contour of the endocardium is traced in each of frames constituting a B-mode tomographic image (black-and-white) produced during a systole, and areas defined with pairs of contour lines are colored differently for display. Consequently, the endocardium is seen making motion in real time with contraction.
However, the foregoing first and second approaches still have the drawbacks described below.
The tissue Doppler imaging system employed in the first approach displays only a Doppler image of a tissue at a certain instant, and is therefore unsuitable for observing a time-sequential change in motion. When a tissue Doppler image is used to assess the cardiac or vascular functions, compared with when a B-mode image is viewed or observed, it is advantageous that velocity information can be observed directly. For determining hypofunction of an organ or discriminating an abnormal region, it takes much time and labor for handling. Moreover, the determination or discrimination still requires expertise and is therefore hard to do.
In the case of the system employed in the second approach, in which tracing a contour in a B-mode image and coloring are combined, there are the drawbacks described below.
First, image information used for imaging is B-mode morphologic information and therefore lacks in motion information of each myocardial region. For example, areas defined pairs of contours of the endocardium in frames are painted in the same color, and the morphology of the endocardium making a time-sequential change in motion is thus expressed. Resultant diagnosis is analogous to that in the foregoing left ventricular analysis, and is achieved by detecting an abnormality such as myocardial infarction through interpretation of a morphologic change of the endocardium. Unless an illness progresses and becomes considerably grave, an apparent morphologic change is usually not discerned. This kind of diagnostic method is therefore unsuitable for early discovery of an illness or discovery of an illness in an early stage.
Secondly, there is a problem with precision in tracing the contour of the endocardium. As mentioned above, if discovery of an abnormal region is dependent only on a morphologic change of the endocardium, the precision in tracing the contour of the endocardium counts. If the precision deteriorates, it would presumably lead to misdiagnosis or missing an abnormal region. Eventually, the reliability of the system deteriorates.